People who become very famous reach a point at which they no longer have to introduce themselves to anyone by name, or say what they do for a living. Most, however, can still remember a time when they were ordinary enough to open a conversation with, "Hello, my name is X and my job is Y." I ask Ant and Dec when those words last passed their lips, and they are both floored.

"In this country?" Ant blinks. "God, I don't know. I honestly don't know. I haven't for so long." Dec thinks hard. "Honestly, I can't think of when it would have been."

Anthony McPartlin and Declan Donnelly have been on our screens for so long that, the first time we meet, I feel as if I've climbed inside my television set. Sheer ubiquity has a lot to do with it: child stars of the teen soap Byker Grove back in the late 1980s, they then graduated into a pop act, PJ and Duncan, in the 1990s, before presenting a new ITV Saturday morning show, SM:TV Live, which was supposed to be for kids but turned into one of those surprise crossover hits that grown-ups fall in love with, propelling the pair into prime-time celebrity.

For the past decade, they have been the face of almost every ITV family entertainment blockbuster, from Pop Idol to Britain's Got Talent to I'm A Celebrity… Get Me Out Of Here!, and even if you have never watched a single one of those shows, I bet you'd still know their faces. Now in their late 30s, they continue to look weirdly boyish, like middle-aged children.

By the laws of show business, at least one of them should have succumbed to the traditional hazards of child stardom – drink and drugs, sexual transgression, monstrous egomania. Yet, with the solitary exception of a drunken night involving Dec and a lap dancer, which ended up in the tabloids, the pair have been almost freakishly clean. Have they never even tried taking drugs?

"Years ago, yeah," Ant admits, "but we're not really druggy people, that's the thing. I think you either go into that crowd as a kid or you don't, and we didn't. We found the love of alcohol very early on and we stayed with it." Laughing, Dec adds, "There's a real pub culture where we're from in Newcastle, so we're just more boozy people."

If one had ever been at risk of self-destruction, though, who was the likelier candidate? Without hesitation, both point at Ant. "Probably me, yeah," he admits. Dec points out affectionately: "There's nothing like the love of a good woman, though."

Ant has been married for six years to his childhood sweetheart, Lisa Armstrong, a makeup artist – but I wonder whether the love of Dec has had a lot to do with it, too? "Yeah, I think that's exactly right," Ant says. "That's the handbrake. With us two, what we do affects somebody else – directly affects somebody else – and what that means is you've got a responsibility. And that's good to have."

I meet Ant and Dec on several occasions over several months, in production meetings, on film sets and photoshoots, and they are the only famous people I have ever known to be exactly – exactly – the same as their public persona. After a while, I become a bit obsessed with trying to work out what is real. Is their on-screen charm a telegenic invention, which they've trained themselves to keep up off-camera? In the end, there is only one possible conclusion: Ant and Dec owe their success in an industry built on artifice to – of all things – authenticity.

To viewers less cynical than me, this may not come as such a surprise. But even the presenters' most loyal fans probably don't realise how much more there is to them than likability – which is largely why they decided to let me watch them at work. "Most people assume that we turn up on a Saturday and make it up as we go along," Ant reflects. "That's quite a compliment, in a way."

Many of Ant and Dec's programmes are made by their own production company, and they are about to revive the one of which they have always been proudest – Saturday Night Takeaway, a variety show they created and executive produced, which ran for nine series from 2002 to 2009. An anarchic comedy rollercoaster of stunts, sketches and competitions, it was both emphatically old-fashioned and ironically knowing. When I sit in on a production meeting, they talk through every tiny detail until I'm no longer sure how much of it comes down to decades of experience and how much to some sort of sixth sense for what an eight-year-old and his octogenarian granny would both want to watch.

Saturday Night Takeaway may be their pride and joy, but it was also the show that very nearly destroyed them. In 2007 it emerged that some of the phone-ins had been rigged, and viewers had been calling premium-rate numbers to win prizes that had already gone. The presenters were cleared of any complicity in the deceit, but at the time they were both certain it was curtains for their career. "Yeah, God, yeah." Ant shudders. "Completely, absolutely, yeah, yeah, yeah," Dec agrees. "That was just the worst, worst period ever."

Ant with a blond Dec in 1996. Photograph: Sportsphoto/Allstar

Best friends since Byker Grove, for years they were flatmates, and even now they live just three doors apart in west London, constantly texting each other while watching TV and finishing each other's sentences like an old married couple. "Fundamentally," Ant says, "we've got the same relationship you see on TV, and I think that's what keeps us sane, because if we had pretended to be somebody else for this length of time, it would have all gone tits up by now."

Do they envy anything about each other? "I guess, for me, it's that he's married and settled with a wife," Dec says.

"Well, I envy your singleness," Ant teases, then feigns panic. "How's that going to go down when I get home?"

Such is their professional co-dependence that they have even taken out life insurance on each other, acknowledging that for all they know, as individual presenters, they could be out of a job for ever. I wonder whether the fear of what they'd risk if they were ever to fall out feels oppressive – like the golden handcuffs of being married to a millionaire? "I don't think so," Ant says at once. "No, I think it's more than that," Dec confirms. "I think if we were to row and to fall out, I'd miss having my best mate more than I'd miss my career. Genuinely, I do. I think almost everything that we do is built on our friendship, not on our career – so we have a career that's built on a friendship, not a friendship built on a career."

It's hard to see how their career could get much bigger in this country, but when I ask why they have never managed to make much of an impact in America, it's the only time their equanimity wobbles. "I'll tell you what's annoying about America," Ant launches in. "We could be here a while," Dec murmurs. "No," Ant goes on with feeling, "what's annoying is we've launched a lot of shows like Pop Idol, and then it goes to the States, and everything stays the same, yet they change the hosts. I'm A Celebrity has been done twice in America now – but they changed the hosts. America's Got Talent, we don't host – somebody else hosts. That becomes very annoying, because I'll tell you what: give us a fair crack at it." "Beginning to get a complex," Dec teases under his breath. "No, give us a fair crack at it and we'll do a good job. But we can't just keep traipsing over there and doing the odd shows."

In 2008 they made a prime-time celebrity gameshow for ABC, Wanna Bet, which looked likely to work, but flopped. "For us to go over there again, I think it will have to be quite a big show, because it's a lot of work and we've got a lot of work on here. And the shows we love that we do over here, we haven't been given the chance." Why does he think they haven't? "I've got no idea." Could it be their geordie accent? "I don't think so," Dec says. "We have got offered some shows in America, but really dopey shows – like reality shows. Horrible. Well, not horrible, but…"

"Just shows we wouldn't do here," Ant says.

"Yeah." Dec nods. "And that was always our thing: if we wouldn't do it at home, we shouldn't do it there. We're not that desperate to break America. And the problem is there's a fear culture there when it comes to their jobs: nobody wants to make the wrong decision, so therefore nobody makes a decision."

"It's not that everyone there lies," Ant adds, "it's just that there's an awful lot of bullshit. And we're not used to working like that – we're used to working with our instinct."

On I'm A Celebrity… Get Me Out of Here! in 2006. Photograph: ITV/Rex Features

Part of the problem could be that the Brits loved by America tend to be pantomime villains – Simon Cowell, Piers Morgan, Jeremy Kyle – and nastiness is the last thing Ant and Dec are cut out for. "Probably, probably," Ant agrees. "I think we're two working-class lads, which could go for us if you pitch the right show and you shoot it in the right place. In LA or New York, we don't fit. I think middle America could relate to us a lot, but we've never been given the opportunity to get out there."

"The thing is," Dec grins, "cool is just something we've never been." He's not wrong – but a lot of uncool stars suffer from what could be called Robbie Williams syndrome, despising the middle-market mass appeal to which they owe their success. Have they never hankered for the glamour of edginess?

"I'm trying to think about it and being as honest as possible," Dec says. "And I think, you know, no. I've never massively minded not being cool."

"Because it's so fleeting, anyway, if you get it," Ant goes on. Exactly, nods Dec. "There's no longevity in cool. So I never want to be cool."

They've never coveted the prestige of the BBC, either, nor longed for the status of grandee presenters who get to cover, say, a royal jubilee? "Not really," Dec says. "I think probably because we grew up on council estates in the north-east, we both kind of came from ITV families, so we never really had this inherent snobbishness that ITV is trashy. We're just… I guess we'll call it ITV heartland viewers, really."

They'd both always voted Labour until the last election, when for the first time Ant voted Conservative. "Which would make a lot of my family up north very angry. I haven't actually told them, so when they read this they'll go, 'What?' But they certainly couldn't give an argument for Labour for me at the moment – not a valid one. Then again, I'd struggle to give an argument for voting Conservative at the moment."

Both their families still live in Newcastle, and when they go back the poverty they see reminds them a lot of what it was like growing up in the 1980s. "It was bleak then," Dec says. "Yeah, it really was bleak, yeah."

Part of their popularity has always been the charm of working-class boys done good, but there was nothing romantic about the poverty Ant recalls. Dec is the youngest of seven children, while Ant grew up with a single mother. "And the one thing that kind of speaks, to my mind, was I rarely saw my mum because she had so many jobs. So when I would come home from school, me and my sister… I would cook for her, or we would kind of sort each other out, because Mum had to get another job to make ends meet. We would have people knocking at the door – money lenders, all that kind of stuff. Yeah, I remember all of that quite vividly and that sticks with you. That's probably one of the reasons we're never too flash with our cash. I think you can show off too much, and that would have been such a no-no from me growing up."

Even though they insist the reports that they are each worth more than £60m are wildly exaggerated, they're still two of the highest-paid presenters in the country. But Dec seems to mean it when he says, "There's that inherent belief that somebody's going to knock on the door tomorrow and say, 'Oh, actually, there's been a mistake and we need it all back.'"

The bigger surprise still comes when I ask about I'm A Celebrity. Their advice to everyone who asks if they'd recommend going on as a contestant is always the same: only do it if you're a nice person or you're going to be horribly exposed. So if they were minor-league celebrities instead of the show's presenters, would they consider it? Dec doesn't pause to think. "No. Because I'm not sure I'm confident in the viewing public seeing every side of me, and I'm not sure I'm confident enough in myself that I'd allow myself to be that exposed." He's being perfectly serious, and for a moment I'm thrown. Have I been wrong all along and in fact they're just better than anyone else on TV at faking authenticity? Then Ant points out, "Let's be honest: most people who are arseholes think they're nice people" and of course the converse is also true.

If all goes to plan, at 7pm tonight Ant and Dec will look like a couple of mates having a bit of a laugh on a Saturday night. "It's the whole swan analogy, isn't it?" Ant says. "It looks graceful on the top, but it's going like the clappers underneath." I believe him when he says they don't care what the TV critics say about it – "That's never really bothered us" – but when I ask if that means they don't suffer from nerves, they look at me as if I must be mad.

"Do I worry?" Ant says. "I shit my pants about it." Already, Dec is having sleepless nights. "Hairs on the back of my neck stand on end when I think about it. I get goosebumps. I'm shitting my pants, yeah."